1. Field of the Invention
This invention relates to an image display device, which comprises substrates opposed to each other and a spacer assembly located between the substrates, and a method of manufacturing the spacer assembly.
2. Description of the Related Art
In recent years, there have been demands for image display devices for high-grade broadcasting or high-resolution versions therefor, which require higher screen display performance. To meet these demands, the screen surface must be flattened and enhanced in resolution. At the same time, the devices must be lightened in weight and thinned.
Flat image display devices, such as a field-emission display (hereinafter, referred to as an FED), have been watched as image display devices that meet the aforesaid demands. The FED has a first substrate and a second substrate that are opposed to each other with a fixed gap between them. These substrates have their respective peripheral edge portions joined together directly or by means of a sidewall in the form of a rectangular frame, and constitute a vacuum envelope. Phosphor layers are formed on the inner surface of the first substrate, while a plurality of electron emitting elements, for use as electron emission sources that excite the phosphor layers to luminescence, are provided on the inner surface of the second substrate.
A plurality of spacers for use as support members are arranged between the first substrate and the second substrate in order to support an atmospheric load that acts on these substrates. In displaying an image in this FED, an anode voltage is applied to the phosphor layers so that electron beams emitted from the electron emitting elements are accelerated by the anode voltage and collided with the phosphor layers, whereupon the phosphor glows and displays the image.
According to the FED constructed in this manner, the size of each electron emitting element is of the micrometer order, and the distance between the first substrate and the second substrate can be set in the millimeter order. Thus, the image display device, compared with a cathode-ray tube (CRT) that is used as a display of an existing TV or computer, can enjoy higher resolution, lighter weight, and smaller thickness.
In order to obtain practical display characteristics for the image display device described above, a phosphor that resembles that of a conventional cathode-ray tube is used, and its anode voltage must be set to several kV or more, and preferably to 10 kV or more. In view of the resolution, the properties and productivity of the support members, etc., the gap between the first substrate and the second substrate cannot be made very wide and is set to about 1 to 2 mm. If electrons that are accelerated at a high acceleration voltage collide with the phosphor screen, moreover, secondary electrons and reflected electrons are generated on the phosphor screen.
If the space between the first substrate and the second substrate is narrow, the secondary electrons and the reflected electrons generated on the phosphor screen collide with the spacers arranged between the substrates, whereupon the spacers are electrified. With the acceleration voltage in the FED, the spacers are positively charged in general. In this case, the electron beams that are emitted from the electron emitting elements are attracted to the spacers and deviated from their original orbits, inevitably. Thus, there is a problem that the electron beams undergo mislanding on the phosphor layers, so that the color purity of displayed images is degraded.
In order to reduce the attraction of the electron beams by the spacers, the whole or part of the spacer surface may possibly be subjected to conductivity treatment to be de-electrified. Described in U.S. Pat. No. 5,726,529, for example, is a structure that subjects second-substrate-side end portions of insulating spacers to conductivity treatment, thereby de-electrifying the spacers.
If the second-substrate-side end portions of the insulating spacers are subjected to conductivity treatment, however, electric charge on the electrified spacers is discharged to a second substrate, so that electron emitting elements on the second substrate may possibly be damaged or degraded to lower the image quality level. Further, reactive current that flows from a first substrate to the second substrate through the spacers increases, thereby causing an increase in temperature or power consumption.